was rather severe in that linear-phase FIR implementations only exist
in the two-tap case
,
. In addition
to relaxing this constraint, we need to be able to design an
-channel filter bank for any
.

The Pseudo-QMF (PQMF) filter bank is a ``near perfect
reconstruction'' filter bank in which aliasing cancellation occurs
only between adjacent bands [194,287]. The PQMF
filters commonly used in perceptual audio coders employ bandpass
filters with stop-band attenuation near dB, so the neglected
bands (which alias freely) are not significant. An outline of the
design procedure is as follows:

Design a lowpass prototype window,
, with length
,

The lowpass design is
constrained to give aliasing cancellation in neighboring subbands:

The filter bank analysis filters
are cosine modulations of
:

(12.98)

, where the phases are restricted according to

(12.99)

again for aliasing cancellation.

Since it is an orthogonal filter bank by construction,
the synthesis filters are simply the time-reverse of the analysis filters:

(12.100)

This PQMF filter bank is reportedly used in MPEG audio, layers I and
II with
bands and
taps (
).